1. Field of the Invention
This invention relates to a method and apparatus of driving a plasma display panel, and more particularly to a method and apparatus of driving a plasma display panel that is capable of driving a plasma display panel at a higher speed as well as improving the contrast. Further, this invention relates to the method and apparatus of driving the plasma display panel that is adaptive to carrying out not only selective write but also selective erase in a predetermined period, that is capable of increasing a driving margin upon the selective write and selective erase and making an initialization stable upon the selective erase in the event that the selective write and the selective erase are all carried out in one frame period.
2. Description of the Related Art
Generally, a plasma display panel (PDP) radiates a phosphorus by an ultraviolet with a wavelength of 147 nm generated during a discharge of He+Xe or Ne+Xe gas to thereby display a picture including characters and graphics. Such a PDP is easy to be made into a thin-film and large-dimension type. Moreover, the PDP provides a very improved picture quality owing to a recent technical development. Particularly, a three-electrode, alternating current (AC) surface-discharge type PDP has advantages of a low-voltage driving and a long life in that it can lower a voltage required for a discharge using wall charges accumulated on the surface thereof during the discharge and protect the electrodes from a sputtering caused by the discharge.
Referring to FIG. 1, a discharge cell of the three-electrode, AC surface-discharge PDP includes a scanning electrode 30Y and a sustaining electrode 30Z formed on an upper substrate 10, and an address electrode 20X formed on a lower substrate 18.
The scanning electrode 30Y and the sustaining electrode 30Z include a transparent electrode 12Y or 12Z, and a metal bus electrode 13Y or 13Z having a smaller line width than the transparent electrode 12Y or 12Z and provided at one edge of the transparent electrode, respectively. The transparent electrodes 12Y and 12Z are formed from indium-tin-oxide (ITO) on the upper substrate 10. The metal bus electrodes 13Y and 13Z are formed from a metal such as chrome (Cr), etc. on the transparent electrodes 12Y and 12Z so as to reduce a voltage drop caused by the transparent electrodes 12Y and 12Z having a high resistance. On the upper substrate 10 provided with the scanning electrode 30Y and the sustaining electrode 30Z, an upper dielectric layer 14 and a protective film 16 are disposed. Wall charges generated upon plasma discharge are accumulated in the upper dielectric layer 14. The protective film 16 protects the upper dielectric layer 14 from a sputtering generated during the plasma discharge and improves the emission efficiency of secondary electrons. This protective film 16 is usually made from MgO. The address electrode 20X is formed in a direction crossing the scanning electrode 30Y and the sustaining electrode 30Z. A lower dielectric layer 22 and barrier ribs 24 are formed on the lower substrate 18 provided with the address electrode 20X. A phosphorus layer 26 is coated on the surfaces of the lower dielectric layer 22 and the harrier ribs 24. The barrier ribs 24 are formed in parallel to the address electrode 20X to divide the discharge cell physically and prevent an ultraviolet ray and a visible light generated by the discharge from being leaked into the adjacent discharge cells. The phosphorus layer 26 is excited and radiated by an ultraviolet ray generated upon plasma discharge to produce a red, green or blue color visible light ray. An inactive mixture gas, such as He+Xe or Ne+Xe, for a gas discharge is injected into a discharge space defined between the upper/lower substrate 10 and 18 and the barrier ribs 24.
Such a three-electrode AC surface-discharge PDP drives one frame, which is divided into various sub-fields having a different emission frequency, so as to realize gray levels of a picture. Each sub-field is again divided into a reset period for uniformly causing a discharge, an address period for selecting the discharge cell and a sustaining period for realizing the gray levels depending on the discharge frequency. When it is intended to display a picture of 256 gray levels, a frame period equal to 1/60 second (i.e. 16.67 msec) in each discharge cell 1 is divided into 8 sub-fields SF1 to SF8 as shown in FIG. 2. Each of the 8 sub-field SF1 to SF8 is divided into a reset period, an address period and a sustaining period. The reset period and the address period of each sub-field are equal every sub-field, whereas the sustaining period and the discharge frequency are increased at a ration of 2n (wherein n=0, 1, 2, 3, 4, 5, 6 and 7) at each sub-field. Since the sustaining period becomes different at each sub-field as mentioned above, the gray levels of a picture can be realized.
Such a PDP driving method is largely classified into a selective writing system and a selective erasing system depending on an emission of the discharge cell selected by the address discharge.
The selective writing system turns off the full screen in the reset period and thereafter turns on the discharge cells selected by the address discharge. In the sustaining period, a discharge of the discharge cells selected by the address discharge is sustained to display a picture.
In the selective writing system, a scanning pulse applied to the scanning electrode 30Y has a pulse width set at 3 μs or more to form sufficient wall charges within the discharge cell.
If the PDP has a resolution of VGA (video graphics array) class, it has total 480 scanning lines. Accordingly, in the selective writing system, an address period within one frame requires total 11.52 ms when one frame period (i.e., 16.67 ms) includes 8 sub-fields. On the other hand, a sustaining period is assigned to 3.05 ms in consideration of a vertical synchronizing signal Vsync. Herein, the address period is calculated by 3 μs (a pulse width of the scanning pulse)×480 lines×8(the number of sub-fields) per frame. The sustaining period is a time value (i.e., 16.67 ms−11.52 ms−0.3 ms−1 ms−0.8 ms) subtracting an address period of 11.52 ms, once reset period of 0.3 ms, and an extra time of the vertical synchronizing signal Vsync of 1 ms and an erasure period of 100 μs×8 sub-fields from one frame period of 16.67 ms.
The PDP may generate a pseudo contour noise from a moving picture because of its characteristic realizing the gray levels of the picture by a combination of sub-fields. If the pseudo contour noise is generated, then a pseudo contour emerges on the screen to deteriorate a picture display quality. For instance, if the screen is moved to the left after the left half of the screen was displayed by a gray level value of 128 and the right half of the screen was displayed by a gray level value of 127, a peak white, that is, a white stripe emerges at a boundary portion between the gray level values 127 and 128. To the contrary, if the screen is moved to the right after the left half thereof was displayed by a gray level value of 128 and the right half thereof was displayed by a gray level value of 127, then a black level, that is, a black stripe emerges on at a boundary portion between the gray level values 127 and 128.
In order to eliminate a pseudo contour noise of a moving picture, there has been suggested a scheme of dividing one sub-field to add one or two sub-fields, a scheme of re-arranging the sequence of sub-fields, a scheme of adding the sub-fields and re-arranging the sequence of sub-fields, and an error diffusion method, etc. However, in the selective writing system, the sustaining period becomes insufficient or fails to be assigned if the sub-fields are added so as to eliminate a pseudo contour noise of a moving picture. For instance, in the selective writing system, two sub-fields of the 8 sub-fields are divided such that one frame includes 10 sub-fields, the display period, that is, the sustaining period becomes absolutely insufficient. If one frame includes 10 sub-fields, the address period becomes 14.4 ms, which is calculated by 3 μs (a pulse width of the scanning pulse)×480 lines×10(the number of sub-fields) per frame. On the other hand, the sustaining period becomes −0.03 ms (i.e., 16.67 ms−14.4 ms−0.3 ms−1 ms−1 ms) which is a time value subtracting an address period of 14.4 ms, once reset period of 0.3 ms, an erasure period of 100 μs×10 sub-fields and an extra time of the vertical synchronizing signal Vsync of 1 ms from one frame period of 16.67 ms.
In such a selective writing system, a sustaining period of about 3 ms can be assured when one frame consists of 8 sub-fields, whereas it becomes impossible to assure a time for the sustaining period when one frame consists of 10 sub-fields. In order to overcome this problem, there has been suggested a scheme of divisionally driving one field, However, such a scheme raises another problem of a rise of manufacturing cost because it requires an addition of driver IC's.
A contrast characteristic of the selective writing system is as follows. In the selective writing system, when one frame consists of 8 sub-fields, a light of about 300 cd/m2 corresponding to a brightness of the peak white is produced if a field continues to be turned on in the entire sustaining period of 3.05 ms. On the other hand, if the field is sustained in a state of being turned on only in once reset period and being turned off in the remaining period within one frame, a light of about 0.7 cd/m2 corresponding to the black is produced. Accordingly, a darkroom contrast ratio in the selective writing system has a level of 430:1.
The selective erasing system makes a writing discharge of the full screen in the reset period and thereafter turns off the discharge cells selected in the address period. Then, in the sustaining periods, only the discharge cells having not selected by the address discharge are sustaining-discharged to display a picture.
In the selective erasing system, a selective erasing data pulse with a pulse width of about 1 μs is applied to the address electrode 20X so that it can erase wall charges and space charges of the discharge cells selected during the address discharge. At the same time, a scanning pulse with a pulse width of 1 μs synchronized with the selective erasing data pulse is applied to the scanning electrode 30Y.
In the selective writing system, if the PDP has a resolution of VGA (video graphics array) class, then an address period within one frame requires only total 3.84 ms when one frame period (i.e., 16.67 ms) consists of 8 sub-fields. On the other hand, a sustaining period can be sufficiently assigned to about 10.73 ms in consideration of a vertical synchronizing signal Vsync. Herein, the address period is calculated by 1 μs (a pulse width of the scanning pulse)×480 lines×8(the number of sub-fields) per frame. The sustaining period is a time value (i.e., 16.67 ms−3.84 ms−0.3 ms−1 ms−0.8 ms) subtracting an address period of 3.84 ms, once reset period of 0.3 ms, and an extra time of the vertical synchronizing signal Vsync of 1 ms and an entire writing time of 100 μs×8 sub-fields from one frame period of 16.67 ms. In such a selective erasing system, since the address period is small, the sustaining period as a display period can be assured even though the number of sub-fields is enlarged. If the number of sub-fields SF1 to SF1 within one frame is enlarged into ten as shown in FIG. 3, then the address period becomes 4.8 ms calculated by 1 μs (a pulse width of the scanning pulse)×480 lines×10(the number of sub-fields) per frame. On the other hand, the sustaining period becomes 9.57 ms which is a time value (i.e., 16.67 ms−4.8 ms−0.3 ms−1 ms−1 ms) subtracting an address period of 4.8 ms, once reset period of 0.3 ms, an extra time of the vertical synchronizing signal Vsync of 1 ms and the entire writing time of 100 μs×10 sub-fields from one frame period of 16.67 ms. Accordingly, the selective erasing system can assure a sustaining period three times longer than the above-mentioned selective writing system having 8 sub-fields even though the number of sub-fields is enlarged into ten, so that it can realize a bright picture with 256 gray levels.
However, the selective erasing system has a disadvantage of low contrast because the full screen is turned on in the entire writing period.
In the selective erasing system, if the full screen continues to be turned on in the sustaining period of 9.57 ms within one frame consisting of 10 sub-fields SF1 to SF10 as shown in FIG. 3, then a light of about 300 cd/m2 corresponding to a brightness of the peak white is produced. A brightness corresponding to the black is 15.7 cd/m2, which is a brightness value of 0.7 cd/m2 generated in once reset period plus 1.5 cd/m2×10 sub-fields generated in the entire writing period within one frame. Accordingly, since a darkroom contrast ratio in the selective erasing system is equal to a level of 950:15.7=60:1 when one frame consists of 10 sub-fields SF1 to SF10, the selective erasing system has a low contrast. As a result, a driving method using the selective erasing system provides a bright screen owing to an assurance of sufficient sustaining period, but fails to provide a clear screen and a feeling of blurred picture due to a poor contrast.
In order to overcome a problem caused by such a poor contrast, there has been suggested a scheme of making an entire writing only once per frame and taking out the unnecessary discharge cells every sub-field SF1 to SF10. However, this scheme has a problem of poor picture quality in that next sub-field can not be driven until the previous sub-field has been turned on and thus the number of gray levels becomes merely the number of sub-fields plus one. In other words, if one frame includes 10 sub-fields, then the number of gray level become eleven as represented by the following table:
TABLE 1GraySF1SF2SF3SF4SF5SF6SF7SF8SF9SF10level(1)(2)(4)(8)(16)(32)(48)(48)(48)(48)0xxxxxxxxxx1∘xxxxxxxxx3∘∘xxxxxxxx7∘∘∘xxxxxxx15∘∘∘∘xxxxxx31∘∘∘∘∘xxxxx63∘∘∘∘∘∘xxxx111∘∘∘∘∘∘∘xxx159∘∘∘∘∘∘∘∘xx207∘∘∘∘∘∘∘∘∘x255∘∘∘∘∘∘∘∘∘∘ 
in Table 1, ‘SFx’ means the x-numbered sub-field and ‘(y)’ expresses a brightness weight set for the subject sub-filed as a decimal number y. Further, ‘O’ represents a sate in which the subject sub-field is turned on while ‘x’ does not state in which the subject sub-filed is turned-off.
In this case, since only 1331 colors are expressed by all combination of refd, green and blue colors, color expression ability becomes considerably insuffiecient in comparison to true colors of 16,700,000. The PDP adopting such a system has a darkroom contrast ratio of 430:1 by a peak white of 950 cd/m2 when the full screen is turned on in the display period of 9.57 ms and a black of 2.2 cd/m2 which is a brightness value adding 0.7 cd/m2 generated in once reset period to 1.5 cd/2 generate in once entire writing period.
As described above, in the conventional PDP driving method, the selective writing system fails to make a high-speed driving because each of a data pulse for selectively turning on the discharge cells in the address period and a scanning pulse has a pulse width of 3 μs or more. The selective erasing system has an advantage of a higher speed driving than the selective writing system because each of a data pulse for selectively turning off the discharge cells and a scanning pulse is about 1 μs, whereas it has a disadvantage of a worse contrast than the selective writing system because the discharge cells in the full screen is turned on in the reset period, that is, the non-display period.